7mm To 5/16 Stem Valve Conversion?

Reply Subscribe

Thread Tools

Search this Thread

Nov 12, 2021 | 02:54 PM

TA0502

Thread Starter

6th Gear

Joined: Apr 2021

Posts: 10

Likes: 1

From: Canada

7mm To 5/16 Stem Valve Conversion?

So I'm porting & polishing a set of heads for my 96ci to throw on when I do the 107ci big bore. I'm reading a bit about upgrading from the 7mm stem valves to a 5/16 stem and was just wondering if basically all that's needed is to order 99-04 guides/seals/springs & valves?

Nov 12, 2021 | 03:41 PM

marcodarq

Grand HDF Member

Joined: May 2014

Posts: 3,843

Likes: 1,174

From: florida

this is not an upgrade. it should only be considered if you are going p to a 2.00 or larger intake valve on a maximum effort build.
m

Nov 12, 2021 | 04:01 PM

TA0502

Thread Starter

6th Gear

Joined: Apr 2021

Posts: 10

Likes: 1

From: Canada

So there's not a while lot of point of going to 5/16 even if you're changing out everything anyway? I just noticed that both Fuelmoto and Zippers list 5/16 valves in their 1.9/1.61 heads, that's what size valves I'm going with so I was debating stepping up the stem size.

Nov 12, 2021 | 07:33 PM

Ed Ramberger

Grand HDF Member

Joined: May 2015

Posts: 3,584

Likes: 1,213

From: Then Wisconsin, now North Carolina

Quote:

Originally Posted by TA0502

Thicker stem is just more mass to control. The 7mm valve uses a beehive spring which further reduces reciprocating mass as opposed to double sprung 5/16 valves.

Nov 13, 2021 | 07:44 AM

wfolarry

Tourer

Joined: Apr 2007

Posts: 412

Likes: 281

From:

I don't consider going to a 5/16 valve an upgrade either.

Nov 13, 2021 | 12:17 PM

Max Headflow

Seasoned HDF Member

Joined: Dec 2005

Posts: 19,930

Likes: 8,033

From: poway

On 06 up heads, you can get a good 20% improvement on them but boring the intake seat out about 0.020, a little clean up and AVnV 1.850 intake valves. Need to recut the seat to center on the valve and outside cut 37 deg. Don't polish the intake port. You'll loose laminar flow. Stay with 7mm.

Nov 13, 2021 | 12:27 PM

TwiZted Biker

Club Member

Joined: Mar 2009

Posts: 66,126

Likes: 51,456

From: Niles Canyon Ca.

7mm = .2755" , 5/16" = .3125", as been mentioned unless this is a max effort build there is a lot of money & labor involved for little gain on a daily rider.